Electric power companies require workers who work in a radiation controlled area, such as a nuclear plant or the like to carry a small-sized electronic radiation counter such as Geiger counter or dosimeter which sets off an alarm when the amount of radiation of each plant worker exceeds a permitted limit. Worker normally work while monitoring the amount of radiation and if the Geiger counter or dosimeter alarms goes off, workers must immediately suspend their work and move away from the site. PLT 1 describes therein a gas density measurement system, for example, as a technique for assisting such a work.